In the production of foam plastic blocks, the desired rectangular cross-section has been achieved by covering the foaming mixture with a sheet of paper or plastic material and avoiding the occurrence of a crown by applying the gentlest possible flattening pressure in the region of the expansion of the foam. However, it has been previously recognized that the surface of the foam plastic block, after the removal of the covering sheet, has large open and closed bubbles. Hitherto it had been thought that these cells were caused by the development of gas during the foaming reaction. For this reason attempts were made to utilize a porous or perforated covering sheet. A disadvantage of this arrangement, however, is that any levelling device used is contaminated in the region of foaming by the reaction mixture penetrating through the covering sheet. If a covering sheet with lower permeability is used, the penetration of the reaction mixture is avoided, but the gas located underneath the covering sheet cannot escape quickly enough through the covering sheet. A combination of two sheets has therefore been suggested, a gas-permeable sheet being covered with a liquid impermeable sheet to prevent the contamination of the levelling device. Naturally, this arrangement is of greater complexity.
An object of the present invention is therefore to solve the problem of the formation of bubbles on the surface of the foam plastic block.